The primary problem that plagued the MBTA’s subway cars this week — and caused thousands of commuters to be stranded on Monday and Tuesday — is a familiar challenge to transit specialists that other cities solved years ago using modern technology.

Many of the stalled trains failed because their motors run on direct current, or DC, power, which malfunctions easily in light, fluffy snow like the more than 40 inches that has blanketed Boston in the past two weeks, MBTA officials say. Transit systems around the country have upgraded to newer alternating current motors, which withstand moisture far better.

Dozens of traction motors — a key part of every train’s propulsion system that keeps the cars moving — failed, forcing the MBTA to take many of its trains off line. The agency operated just half of its Red and Orange Line fleets for periods earlier this week.

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Beverly A. Scott, the general manager of the T, blamed a lack of investment for keeping the older motors.

“If, years ago, the authority had had the money and funds to do a midlife overhaul and replace and upgrade the propulsion system, the traction motors would have been updated with them,” she said, “But some things you don’t get to pick and choose.”

Scott added that new Red and Orange Line cars would provide more reliable rides in the future.

“We’re not going to make excuses, but that’s just the reality,” she said.

Of the T’s 218 Red Line cars, 132 are at least 25 years old and run on DC motors. The newest Red Line cars, a group of 86 that went into service in 1994, run on AC systems.

The Orange Line’s 120 cars cars also run on the old, direct current motors, and haven’t been through any major rehabilitation program since being put online in 1979 through 1981.

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Both alternating and direct current motors use magnetic fields to turn electrical energy into motion, but they are powered by different types of current. Direct current flows in one direction, like the power from a battery; alternating current is what powers your house.

And unlike a locomotive engine that propels a train, each subway car has its own electric propulsion system — that’s why a subway train can accelerate so quickly compared to a freight train.

The T relies on its repair and maintenance workers to replace the old motors. Last week, the T paid $172,000 in overtime costs to make and repair traction motors.

About 85 percent of the breakdowns over the last week stemmed from propulsion system failures, which include traction motor problems, according to the T.

The motors weren’t the only issue wreaking havoc on commutes this week. Icy conditions caused the third rail on the Red Line to lose power on Monday night.

The Massachusetts Bay Transportation Authority plans to replace many of the Red and Orange Line cars, but doing so will take several years. In October, the MBTA chose CNR-MA, a Chinese company that makes rail cars, for a $566.6 million contract to provide 152 new Orange Line cars and up to 190 new Red Line cars, which would replace much of the aged fleet.

Some of the losing bidders on the project are challenging the agency’s choice of CNR-MA, a newcomer to the United States rail market. But if the contract goes through as planned, delivery of the new cars would start in 2018 for the Orange Line cars and 2019 for the Red Line cars, with all expected to be in service by 2023.

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Serious snowfall has crippled the train systems of other cities, but many responded by upgrading their equipment.

Within a year of Chicago’s 1979 blizzard, nearly every traction motor in the Chicago Transit Authority’s fleet failed, according to “CTA at 45,” a 1993 book chronicling the history of the transit authority there. It took until 2005, but a push to replace the aging system with cars with AC motors finally got rolling.

“DC systems, such as the one in use at CTA, are becoming obsolete,” said Frank Kruesi,
then president of the authority in a news release 10 years ago. “Converting to a more modern AC system will improve reliability and reduce the growing cost of maintaining an outdated system.”

The cars began rolling out in 2010.

In Philadelphia, where the fleet of SEPTA cars on the line most similar to the Red Line date to the late ’90s, the AC traction motors aren’t a problem in the snow. But on older commuter rail trains there, the motors routinely fail in the snow. The agency hoards rebuilt DC motors to swap out when those in the old trains fail — what Manny Smith, a SEPTA spokesman, called a “snow bank.”

Boston also keeps spare traction motors on hand, but the onslaught of snow is depleting that stockpile, according to Jeffrey D. Gonneville, the chief mechanical officer of the T.

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In New York, an MTA spokeswoman said the city’s system had experienced no such problems with traction motors. Trains were stored underground during the recent storms, and some were equipped with ice scrapers to clear the third rail.

The average age of New York City subway cars is 19 years, said Amanda Kwan, the MTA spokeswoman. That means the newest cars on Boston’s Red Line are older than the average car in New York.

Heavy snow is a challenge for transit systems, and Boston’s recent storms would have been cause for emergency plans to be enacted wherever they occurred, said Greg Hull of the American Public Transportation Association, of which the MBTA is a member.

But the severity of technical problems rises with age, Hull said. “With some of the older equipment, there’s a higher risk of failure.”

Instead of replacing cars, the T has gone through several efforts to rehabilitate old equipment. About 74 of the oldest Red Line cars, which were built in 1968 and 1969, went through a major rebuilding program in 1985 through 1988.

James O’Leary, the general manager of the MBTA at the time, said the program essentially “gutted” the wiring, electronic system and motors, replacing major components. O’Leary said it was an essential program to keep the cars alive.

The T has also rehabbed 46 of 58 Red Line cars, which were built in 1987.

Though officials say the rehabilitation programs are necessary to keep the cars running, they leave the cars open to the problems accompanied by older motor systems.

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Gonneville said the outdated cars can limit some of the maintenance, likening them to old cars that you “constantly are bringing to the shop and wondering if it’s going to make it out to the Turnpike,” versus a reliable new car.